Back to WW2! Back to WW2 I say!
Not just because it's the last war that
mattered...
Not just because it involved so many countries and shaped the entire world as we know it...
But because if you want more innovative gameplay, more variety in who you play as, what vehicles you wage war with, where you go, and what kind of terrain you experience, then no conflict in history has you covered like WW2 has. Sci-Fi can arguably match it, but it's only ever as good as the writing of it's creators, whereas WW2 is written by actual real events; real decisions made by real people, acting on believable impulses, because they really happened.
World War Two, man. That's where it's at. There has never been -nor will there ever be- a war more perfectly suited to gaming.